The subject of the invention is a solids composition based on clay minerals and use thereof for the preparation of a water-based drilling fluid.
The sinking of drill holes needed for the development and exploitation of fossil fuels, water or geothermal energy, and also for tunnelling under roads and river or stream courses, requires the use of drilling fluids.
This drilling fluid serves many purposes; in addition to stabilizing the drill hole, it serves to cool and lubricate the drill bit, to carry the drill cuttings when it is circulating and to suspend the drill cuttings when it is not circulating. This means that when circulating, the drilling fluid must be pumpable, being characterized by low viscosity and turbulent flow properties, and when not circulating it must be characterized by immediate gelatinous thickening so as to prevent the drill cuttings from sinking to the bottom of the drill hole again. This type of flow behaviour is referred to as shear-thinning, since the viscosity, as proportionality factor between shear stress and shear gradient, decreases with increasing shear gradient.
According to Bingham, the rheology of a drilling fluid can be described by the yield point, YP (lbs/100 ft.sup.2) and the plastic viscosity (PV [cP]). The yield point describes the minimum shear stress required to induce a solid to flow. The further rise in shear stress as the shear gradient increases is termed plastic viscosity. These parameters can be determined by measuring the shear stress in a rotary viscometer with Couette geometry (eg, FANN 35) at different speeds of rotation. The plastic viscosity (PV) is thus obtained as the difference between the shear stresses at 600 and 300 rpm, and the yield point (YP) as the difference between the PV and the shear stress at 300 rpm. The yield point is proportional to the discharging capacity of a drilling fluid. A high plastic viscosity leads to slow drilling progress.
Drilling fluids may be water- or oil-based. In the case of water-based fluids, a clay mineral is usually added to impart viscosity. The disadvantage of such clay-water drilling fluids is that the clay concentration necessary to ensure that the drilling fluid has a sufficiently high carrying capacity results in a very high plastic viscosity. This in turn leads to slow drilling progress and is therefore undesirable.
From the U.S. Pat. Nos. 4,664,843, 5,232,627, 5,532,212 and 5,576,271 additives based on mixed metal hydroxide derivatives are known, which lessen or overcome the aforementioned disadvantages of aqueous drilling fluids. However, the disadvantage of drilling fluids treated with mixed metal hydroxides is that they necessitate a very complicated and tedious mixing procedure, which must be adhered to precisely. First of all, a 2-3 wt. % bentonite suspension has to be prepared. Prior to the addition of other additives, the bentonite must swell for six hours. Then 0.2-0.3 wt. % of mixed metal hydroxide is added, and the suspension sheared, eg, by means of vigorous stirring. The pH must subsequently be adjusted to 10.5 with soda or NaOH, and the mixture then stirred again for another 60 minutes. Finally, mixed metal hydroxide must be added again in order to adjust the flow behaviour, and the pH readjusted. The necessary equipment to do all this is not always available at the drilling site, which severely limits the use of the drilling fluid. There have already been a number of attempts to simplify this procedure, for example by combining the components bentonite, mixed metal hydroxide and base in a dry mixture. The advantage of such a formulation would be that the mixture need only be added to the water, and pH adjustment would be superfluous. The user would have a drilling fluid of high carrying capacity, which would be ready for immediate use.
All previous endeavours in this direction, however, have failed, because after just a few days these formulations no longer show any viscosity-imparting effect. The object of this invention was thus to develop a solids composition based on clay minerals which does not have the aforementioned disadvantages of the prior art and with which it is possible to prepare a water-based drilling fluid of high carrying capacity, even after the dry mixture has been stored for several months.
This object is established by means of a solids composition consisting of a) 40-99.8 wt. % of a clay mineral with a reduced moisture content of .ltoreq.7 wt. %; b) 0.01-30 wt. % of a mixed metal hydroxide derivative, and c) 0.01-30 wt. % of a solid base, and if necessary other additives. Surprisingly, it was found that the solids composition of the invention constitutes a thickening agent for water-based drilling fluids, which is easy to use and has a long-term stability of at least three months.